Effects Of The Emission Layer Structure On The Electroluminescence Performance Of The White Organic Light Emitting Diodes Based On Thermally Activated Delayed Fluorescence Materials

White organic light emitting diodes(WOLED)employing blue,green and red thermally activated delayed fluorescence(TADF)emitters of DMAC-DPS(B),4CzIPN(G)and 4CzTPN-Ph(R)and various emission layer(EML)structures of G/B,G/R/B and R:G(4CzTPN-Ph(R)doped in 4CzIPN(G)host)/B are fabricated and the effects of the EML structure,including EML thickness and 4CzTPN-Ph(R)dopant concentration,on the electroluminescence(EL)performance are investigated.It is found that the WOLED with the simple EML structure of G(20 nm)/B(10 nm)demonstrates maximum power efficiency(PE)around 16 lm/w and color rendering index(CRI)of 81.Meanwhile,R:G/B EML exhibits better EL performance than that of G/R/B EML.Especially,striking high CRI of 90 together with maximum PE around 13 lm/w can be obtained for R(1 wt%):G(20 nm)/B(10 nm)EML by meticulously modulation of EML thickness and 4CzTPN-Ph(R)dopant concentration,indicating the achievement of balance between EL efficiency and CRI for non-doped TADF emitter-based WOLED.Furthermore,the stacked EML of G(20 nm)/R(0.1 nm)and G(20 nm)/R(0.1 nm)/B(10 nm)reveal much more serious obstruction of delayed fluorescence(DF)compared to mixed EML of R(1 wt%):G(20 nm)and R(1 wt%):G(20 nm)/B(10 nm),implying more intensified quenching effects from the sandwiched 0.1 nm 4CzTPN-Ph(R)layer which will lead to deterioration of EL performance.The analysis of transient PL decaying dynamics derives different energy transfer routes in R(1 wt%):G(20 nm)/B(10 nm)and G(20 nm)/R(0.1 nm)/B(10 nm)EML,in which the energy transfer from DMAC-DPS(B)to 4CzTPN-Ph(R)can be ignored in R:G/B while it plays a key role in the EL of G(20 nm)/R(0.1 nm)/B(10 nm).Furthermore,the energy transfer in the G(20 nm)/R(0.1 nm)/B(10 nm)EML is remarkably enhanced compared to that in R(1 wt%):G(20 nm)/B(10 nm)EML,both from DMAC-DPS(B)to 4CzIPN(G)and 4CzTPN-Ph(R),and from 4CzIPN(G)to 4CzTPN-Ph(R),leading to the enhancement of PF and hindrance of DF.In virtue of the dominant DF of the TADF emitters,the seriously hindered DF of the stacked G(20 nm)/R(0.1 nm)/B(10 nm)EML will impose detrimental effects on the EL performance of WOLEDs,which make the EML of R(1 wt%):G(20 nm)/B(10 nm)more beneficial to realize the balance between EL efficiency and CRI.Our study provides an alternative way to achieve high quality WOLED via the application of TADF emitter-based EML with simple structure.